Allow build_unit_cell to return atom site labels

[janbridley]

Description

• Added the additional_columns arg to cif.build_unit_cell to allow for replication of element symbols (and other data from the array).
• Ensured consistent ordering of unit cell positions by sorting in order of symmetry operations
• Fixed type hinting in a few docstrings

TODOs

• Allow for additional data return
• Update return types in docstring
• Add tests for new functionality
• Verify this meets use cases

NOTE

parsnip ignores fractional occupancies while building unit cells. This is different than ASE, which always selects the most dominant species. The position data is unaffected, but users extracting additional columns should be aware that the selection of a particular species is not guaranteed.

Motivation and Context

As requested in #37, it would be useful to (optionally) replicate additional data alongside the atomic positions. This functionality has been added with the additional_columns arg in build_unit_cell, which allows returns associated data for each replicate atom. The current code returns an array for the positions and the additional data: while I considered a structured array, doing so makes indexing columns nontrivial.

Multi-array return (this PR)

(array([['Si', '0.01013'],
       ['Si', '0.01013'],
       ...
       ['O', '0.00127'],
       ['O', '0.00127'],
       ['O', '0.00127']], dtype='<U7'), 
array([[0.    , 0.1522, 0.25  ],
       [0.    , 0.8478, 0.75  ],
       ...
       [0.2336, 0.1245, 0.5814],
       [0.7664, 0.8755, 0.4186],
       [0.2664, 0.3755, 0.0814]]))

Structured array (alternate option)

[('Si', '0.01013', 0.    , 0.1522, 0.25  )
 ('Si', '0.01013', 0.    , 0.8478, 0.75  )
 ...
 ('O', '0.00127', 0.2336, 0.1245, 0.5814)
 ('O', '0.00127', 0.7664, 0.8755, 0.4186)
 ('O', '0.00127', 0.2664, 0.3755, 0.0814)]

Resolves #37

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change

Checklist:

• I am familiar with the Development Guidelines
• The changes introduced by this pull request are covered by existing or newly introduced tests.
• I have updated the changelog and added my name to the credits.

[janbridley]

@cajfisher Please let me know if this fulfills your use case! Add additional_columns="_atom_site_label" to your calls to build_unit_cell to get an auxiliary array of the labels. By default, additional_columns is None and the method functions as it did previously.